Remote control or remote access systems use a transmitter and a receiver. An exemplary example is a garage door opener system where a transmitter is contained in a remote control unit and a receiver is connected to a garage door motor. The transmitter and receiver are typically comprised of different integrated circuits. When activated, the transmitter sends a data stream to the receiver. The data stream includes a transmitter identification code which is modulated onto a radio frequency or infra-red carrier. The receiver demodulates and decodes this pattern. If it matches a code stored in its own memory, access is granted.
The safety of a basic system relies on the assumption that only an authorized user could transmit an identification code matching the receiver's code. By using a large enough identification code, chances that two users would accidentially have the same identification code, can be reduced to virtually zero. However, there remains the possibility that an unauthorized user would receive a "valid" identification code by "eavesdropping" and then recreate it to gain unauthorized access. The problem is worse with radio frequency systems, where the transmitted signal can be received by anyone in close enough proximity to the transmitter.
To avoid this possibility, the transmitted code is changed every time the transmitter is activated. Daimler Benz incorporates such a system in one of its Mercedes automobile security systems which uses different integrated circuits manufactured by EURASIL for the transmitter and the receiver. The codes are changed (rolled) according to a fixed sequence, known only to the transmitter and the receiver (algorithm built into the integrated circuit), but unknown to any potential eavesdropper. It unfortunately uses sophisticated complex software to generate the sequence. What is needed is a simple method to change the transmitter code.